This project is designed to delineate sites and mechanisms of hormonal control of renal sodium and potassium transport. Previous studies with isolated perfused outer cortical proximal straight tubules from rabbits demonstrated that chronic administration of deoxycorticosterone acetate (DOCA) increased fluid absorption and cell volume. Further experiments to determine whether these are direct effects of DOCA revealed: (a) maintenance on a low sodium diet did not increase fluid absorption or cell volume. b) DOCA administration to rabbits maintained on a low salt diet did not increase fluid absorption or cell volume. We conclude from this that the DOCA-induced increases in fluid adsorption and cell volume are not due to a direct mineralocorticoid action, but to indirect effects related to factors such as increased extracellular fluid volume, increased glomerular filtration rate or potassium depletion. Electron micrographs of perfused tubules from DOCA-treated and normal rabbits revealed no consistent major morphologic differences.